As known, a variable valve timing system is configured at the engine to vary a valve timing (time for opening and closing an intake valve and an exhaust valve) of the intake valve and the exhaust valve according to engine conditions for enhancing fuel efficiency and output.
Also, as the variable valve timing system controls a valve timing, fuel efficiency and output performance may be enhanced and an emission reduction effect may be improved by lengthening valve overlap (a state in which both an intake valve and an exhaust valve are open) by the variable valve timing system during a catalyst heating section, an internal exhaust gas recirculation (EGR) rate may be increased to increase recombustion combustion of imperfect combustion gas and a temperature of a mixer to increase evaporation of fuel, whereby emission may be reduced.
In this manner, since the variable valve timing system is also related to an exhaust gas reducing device, self-diagnosis based on on-board diagnostics (OBD)-2 regulations is required.
That is, it is required to stably maintain emission reduction performance by diagnosing whether the variable valve timing system has a fault.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.